1. Field of the Invention
This invention relates to photoluminescence and in particular to a method of laser discrimination using stimulated luminescence.
2. Description of the Prior Art
The use of rare earth doped alkaline earth sulfide as an efficient phosphor has been known for some time. However, since the advent of different types of laser sources, there has been renewed interest in using these phosphors as optically stimulated luminescence (OSL) dosimeters and infrared sensors. For these devices, storage phosphors are necessary and doubly doped alkaline earth sulfides are known to be efficient storage phosphors.
A second type of luminescence called thermoluminescence (TL) occurs when a charged phosphor is heated. A charged phosphor is one that has trapped electrons. The heating of the charged phosphor can be achieved by conventional methods such as direct contact of the phosphor with a heated object or by using an infrared beam produced by incandescence. Recently, heating has also been achieved by irradiation with a suitable laser beam which is absorbed directly by the charged phosphor or via a supporting substrate. Heating of a charged phosphor causes detrapping of the trapped charges. The radiative recombinations of the released charges with the opposite charges at the recombination centers causes the thermoluminescence to occur.
A fine discussion of doubly doped alkaline earth sulfides used as storage phosphors can be found in U.S. Pat. No. 4,806,772 issued to Lindmayer. Lindmayer applies the fundamentals of electron trapping in disclosing a process for making an infrared sensing device capable of detecting a known infrared light source. A point of luminescence on the device indicates where the infrared beam is striking the device. Therefore, the device is useful as a detecting tool for scientific or industrial instrumentation.
The limitation of Lindmayer and other prior art stimulated luminescence techniques is that known sources of infrared stimulation are merely detected by the phosphors through luminescence. These prior art stimulation techniques involve the matching of a known laser source to a phosphor known to emit luminescence when stimulated by the known laser source. Unfortunately, lasers being used for the tracking and ranging of moving objects vary in wavelength and intensity. Accordingly, prior art stimulation techniques do not address the problem of laser discrimination using a single phosphor.